1. Field of the Invention
The present invention relates to a stainless steel foil having a high oxidation resistance and good processability and applicable to catalyst-carriers of combustion exhaust gas purifiers.
2. Description of the Related Art
The catalyst-carriers of combustion exhaust gas purifiers of automobiles, etc., are conventionally fabricated of a ceramics honeycomb. An advantageous alternative to the ceramics honeycomb is a heat-resistant stainless steel foil, by which the honeycomb wall thickness as well as the gas flow resistance and the heat capacity can be reduced, to thereby improve the engine performance and save the expensive catalyst metals. For example, Japanese Unexamined Patent Publication (Kokai) Nos. 50-92286, 50-144689 and 57-71898 proposed an Fe-Cr-Al-based heat-resistant metal foil for such a honeycomb structure.
Such an alloy must have specific characteristics, particularly the oxidation resistance and the adhesion of an oxide film thereof, and accordingly, the conventional metal foils are made of an alloy based on an Fe-Cr-Al alloy, and modified to provide an improved oxidation resistance and/or an improved adhesion with a wash coat, i.e, a direct support for catalysts, utilizing the established excellent properties of the Fe-Cr-Al alloy, such as the oxidation resistance and the adhesion of an oxide film, as an electrical heating element and a material to be used for the high temperature members of heating systems Nevertheless, the above-referred Japanese patent publications both use yttrium (Y) to improve the oxidation resistance, and the use of such an expensive element is acceptable only for very limited applications.
Japanese Unexamined Patent Publication (Kokai) No. 63-45351 proposed another modified Fe-Cr-Al-based alloy containing rare earth metals in a total amount of up to 0.06 wt %, including 0.002 to 0.05 wt % of rare earth metals from the group of La, Ce, Pr, and Nd, to prevent the spalling of an oxide film, and further proposed modified alloys supplemented with Zr or Nb in a content specified with respect to the C and N contents, to stabilize carbon and nitrogen in the alloy as a carbide and a nitride or to ensure the high temperature creep strength of the former alloy, respectively. This Japanese patent publication states that a total amount of rare earth metals of more than 0.06 wt % does not substantially improve the oxidation resistance, in comparison with a lower total amount, and further the working of the alloy becomes impossible at the usual hot-working temperatures.
Japanese Unexamined Patent Publication (Kokai) No. 63-45351 also states that the addition of Y to Fe-Cr-Al-based alloys is unacceptably expensive, and proposed that lanthanoid elements (hereinafter simply referred to as "Ln" or "the Ln elements") other than Ce or La alone be added to an alloy in an amount of from 0.05 to 0.20 wt %. This publication states that such a specified composition is based on the phenomenon that the reduction of the hot workability due to the Ln addition is caused by the presence of Ce, which also lowers the oxidation resistance, and therefore, the addition of Ln other than Ce enables an alloy to be hot-worked and improves the oxidation resistance. Nevertheless, the Ln elements are very reactive and have similar chemical properties, and therefore, are difficult to separate from each other and purify, and thus La is in fact much more expensive than mischmetal, which is a generally available mixture of the Ln elements, although La is inexpensive when compared with Y in an essentially pure form. Similarly, the separation and removal of Ce would also unavoidably raise the material price.
Moreover, although the above-mentioned conventional technologies made a study of the adhesion of an oxide film and the oxidation resistance, these publications do not take into consideration the influence of the chemical composition of a component foil of a honeycomb on the structural durability of a catalyst-carrier when used in a high temperature exhaust gas flow.